The microscopic diagnosis of pigmented lesions is based on the presence and localization of atypical melanocytes. Aggregation of atypical melanocytes may be seen in atypical/dysplastic nevi. These may sometimes be difficult to distinguish from superficial spreading malignant melanoma. There are also some more infrequent variants, as for instance desmoplastic melanoma, which may look similar to benign lesions, and thereby may give rise to diagnostic problems. These cases require experience and high professional skill.
For the diagnosis of malignant melanoma several other factors must be evaluated:
- Type of melanoma
- Infiltration of vessels
- Resection margins
- Mitotic rate, or Clark`s level if mitotic rate cannot be determined reliably.
- In addition, possible lymphocyte invasion, presence of necrosis, regression and neurotropism (tumor growth along nerves) should be commented.
Melanomas are divided into four types.
Superficial spreading malignant melanoma
In this tumor type (circa 70%), large atypical melanocytes are often found infiltrating epidermis in cooperation with a component growing into the underlying dermis. These atypical melanocytes become different, often smaller than those in epidermis. The melanocytes in epidermis spread peripheral around the dermal invasive part of the lesion. Thereby the name “superficially spreading.”
Nodular malignant melanoma
In this type (circa 15%) there is no growth of melanocytes in the epidermis outside the invasive part of the tumour. Secondary tumor infiltration may be seen centrally superficial to the lesion.
Lentigo malignant melanoma
In lentigo malignant melanoma (circa 10%) epidermis is thin and atrophic, indicating damage to actin. The melanocytes are spindle-shaped or epitheloid and grow linearly in one or more layers in the epidermis.
Acral lentiginous melanoma
Acral lentiginious melanoma (circa 5%) are observed in the sole of the foot, palm of the hand, toes, fingers and sublingually, the epidermis is thick with elongated rete ridges. In this type of melanoma, large single atypical melanocytes are seen in the basal part of the epidermis.
Amelanotic melanomas (without pigment) are rare and amount to 1-2% of the melanomas. The diagnosis is most frequently made after it has metastasized.
Light microscopic picture of in situ components beside the main tumor in a superficially spreading melanoma. Click to enlarge.
Light microscopic picture of nodular malignant melanoma. No infiltration of epidermis. Click to enlarge.
Light microscopic picture of superficial malignant melanoma infiltrating epidermis. Click to enlarge.
Thickness of tumor (ad modum Breslow)
Tumor thickness is the most important single factor for the histological evaluation. This is measured (in micrometer under the microscope) as the longest diameter from the top of the tumor to the deepest epidermal tumor cell. The length is given in mm with one decimal.
The pathologist must consider whether the pathological slides are representative for the type and extent of the growth, assuming the entire tumor is sent for examination. The slides must include the area with the expected deepest infiltration.
The histologic information is important for the choice of treatment and for the further evaluation of melanoma patients. Satellites in the dermis must be included. The evaluation of the tumor thickness may be uncertain or impossible when regressive changes in the tumor have taken place. In such cases, the length both with and without inclusion of the regressive zone may be given by the pathologist.
The following factors must be included in the pathology report:
- Histological type, invasive or in-situ
- Thickness of tumour, when invading (Breslow) in tenths of mm
- Ulceration (present/not present)
- Other factors (infiltration of vessels, infiltration of lymphocytes)
- Resection margins
For lymph node resection:
- Number of metastatic nodes
- Total number of resected nodes
- Perinodal growth of tumor
- Tumor in the resection margin
In malignant melanomas, a variable microscopic picture is seen. Lymphocyte filtration is less frequent compared to skin lesions. Around 1/3 of the melanomas in the nose and oral cavity are melanotic. This type of melanoma may be difficult to differentiate from other types of cancer in mucosal membranes. Immunohistochemistry may be helpful in this respect.